In the News

A pair of Earth orbiters designed to keep track of the planet's water resources and evolving water cycle is scheduled to launch this month – no earlier than May 22, 2018. The Gravity Recovery and Climate Experiment Follow-On mission, or GRACE-FO, will pick up where its predecessor, GRACE, left off when it completed its 15-year mission in 2017. By measuring changes in Earth’s gravity, the mission will track water movement around the globe, identifying risks such as droughts and floods and revealing how land ice and sea level are evolving. The GRACE-FO mission is a great way to get students asking, and answering, questions about how we know what we know about some of the major components of Earth’s water cycle: ice sheets, glaciers, sea level, and ground-water resources.

How It Works

animated heat map of co2 concentrations on Earth

Earth Science Lessons

Explore a collection of standards-aligned lessons for grades K-12 all about our home planet.

The GRACE-FO mission, a partnership between NASA and the German Research Centre for Geosciences (GFZ), will measure small variations in Earth’s mass to track how and where water is moving across the planet. This is no easy task, as water can be solid, liquid or gas; it can be in plain sight (as in a lake or glacier); it can be in the atmosphere or hidden underground; and it’s always on the move. But one thing all this water has in common, regardless of what state of matter it is in or where it is located, is mass.

Everything that has mass exerts a gravitational force. It is this gravitational force that GRACE-FO measures to track the whereabouts of water on Earth. Most of Earth's gravitational force, more than 99 percent, does not change from one month to the next because it is exerted by Earth’s solid surface and interior. GRACE-FO is sensitive enough to measure the tiny amount that does change – mostly as a result of the movement of water within the Earth system.

GRACE-FO works by flying two spacecraft in tandem around Earth – one spacecraft trailing the other at a distance of about 137 miles (220 kilometers). By pointing their microwave ranging instruments at each other, the satellites can measure tiny changes in the distance between them – within one micron (the diameter of a blood cell) – caused by changes in Earth’s gravitational field. Scientists can then use those measurements to create a map of Earth’s global gravitational field and calculate local mass variations.

As the forward spacecraft travels over a region that has more or less mass than the surrounding areas, such as a mountain or low valley, the gravitational attraction of that mass will cause the spacecraft to speed up or slow down, slightly increasing or decreasing the relative distance between it and its trailing companion. As a result of this effect, GRACE-FO will be able to track water as it moves into or out of a region, changing the region’s mass and, therefore, its gravity. In fact, the previous GRACE spacecraft measured a weakening gravity field over several years in Central California, enabling an estimate of aquifer depletion, and in Greenland, providing accurate measurements of ice melt over more than 15 years.

Find out more about how the mission works in the video below, from JPL's "Crazy Engineering" video series:

Why It’s Important

Tracking changes in our water resources and the water cycle is important for everyone. The water cycle is one of the fundamental processes on Earth that sustains life and shapes our planet, moving water between Earth's oceans, atmosphere and land. Over thousands of years, we have developed our civilizations around that cycle, placing cities and agriculture near rivers and the sea, building reservoirs and canals to bring water to where it is needed, and drilling wells to pump water from the ground. We depend on this cycle for the water resources that we need, and as those resources change, communities and livelihoods are affected. For example, too much water in an area causes dangerous floods that can destroy property, crops and infrastructure. Too little water causes shortages, which require us to reduce how much water we use. GRACE-FO will provide monthly data that will help us study those precious water resources.

Graphic showing the amount of water in aquifers across Earth as measured by GRACE

A map of groundwater storage trends for Earth's 37 largest aquifers using GRACE data shows depletion and replenishment in millimeters of water per year. Twenty-one aquifers have exceeded sustainability tipping points and are being depleted, and 13 of these are considered significantly distressed, threatening regional water security and resilience. Image credit: NASA/JPL-Caltech

Changes to Earth’s water over multiple years are an important indicator of how Earth is responding in a changing climate. Monitoring changes in ice sheets and glaciers, surface and underground water storage, the amount of water in large lakes and rivers, as well as changes in sea level and ocean currents, provides a global view of how Earth’s water cycle and energy balance are evolving. As our climate changes and our local water resources shift, we need accurate observations and continuous measurements like those from GRACE and GRACE Follow-On to be able to respond and plan.

As a result of the GRACE mission, we have a much more accurate picture of how our global water resources are evolving in both the short and long term. GRACE-FO will continue the legacy of GRACE, yielding up-to-date water and surface mass information and allowing us to identify trends over the coming years.

Teach It

Have students interpret GRACE data for themselves:
Get students learning about global water resources:
Teach students to read, interpret and compare “heat map” representations of Earth science data:

Explore More

Try these related resources for students from NASA's Space Place:

TAGS: Earth Science, Teach, In the News, GRACE, Climate Change, Water, Water Cycle

  • Ota Lutz

UPDATE - May 9, 2016: NASA's Solar Dynamics Observatory, or SDO, spacecraft captured stunning images of the May 9, 2016 transit of Mercury. Visit the mission's Transit of Mercury page to see a collection of videos of the transit compiled using SDO images. And have students play "Can You Spot Mercury?" in our educational slideshow. 

In the News

It only happens about 13 times per century and hasn’t happened in nearly a decade, but on Monday, May 9, Mercury will transit the sun. A transit happens when a planet crosses in front of a star. From our perspective on Earth, we only ever see two planets transit the sun: Mercury and Venus. (Transits of Venus are even more rare. The next one won't happen until 2117!) On May 9, as Mercury passes in front of the sun, viewers around Earth (using the proper safety equipment) will be able to see a tiny dark spot moving slowly across the disk of the sun.

CAUTION: Looking directly at the sun can cause permanent vision damage – see below for tips on how to safely view the transit.

Why It's Important

Then and Now

In the early 1600s, Johannes Kepler discovered that both Mercury and Venus would transit the sun in 1631. It was fortunate timing: The telescope had been invented just 23 years earlier and the transits wouldn’t happen in the same year again until 13425. Kepler didn’t survive to see the transits, but French astronomer Pierre Gassendi became the first person to see the transit of Mercury (the transit of Venus wasn’t visible from Europe). It was soon understood that transits could be used as an opportunity to measure the apparent diameter – how large a planet appears from Earth – with great accuracy.

In 1677, Edmond Halley observed the transit of Mercury and realized that the parallax shift of the planet – the variation in Mercury’s apparent position against the disk of the sun as seen by observers at distant points on Earth – could be used to accurately measure the distance between the sun and Earth, which wasn’t known at the time.

Today, radar is used to measure the distance between Earth and the sun with greater precision than can be found using transit observations, but the transit of Mercury still provides scientists with opportunities for scientific investigation in two important areas: exospheres and exoplanets.

Exosphere Science

Some objects, like the moon and Mercury, were originally thought to have no atmosphere. But scientists have discovered that these bodies are actually surrounded in an ultra-thin atmosphere of gases called an exosphere. Scientists want to better understand the composition and density of the gases that make up Mercury’s exosphere and transits make that possible.

“When Mercury is in front of the sun, we can study the exosphere close to the planet,” said NASA scientist Rosemary Killen. “Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there.”

Mercury transits the sun in 2006
This image of Mercury passing in front of the sun was captured on Nov. 8, 2006 by the Solar Optical Telescope, one of three primary instruments on the Hinode spacecraft. Image credit: Hinode JAXA/NASA/PPARC

Exoplanet Discoveries

When Mercury transits the sun, it causes a slight dip in the sun’s brightness as it blocks a tiny portion of the sun's light. Scientists discovered they could use that phenomenon to search for planets orbiting distant stars, called exoplanets, that are otherwise obscured from view by the light of the star. When measuring the brightness of far-off stars, a slight recurring dip in the light curve (a graph of light intensity) could indicate an exoplanet orbiting and transiting its star. NASA’s Kepler mission has found more than 1,000 exoplanets by looking for this telltale drop in brightness.

illustration of how transits are used to find exoplanets
This diagram shows how scientists use the light curves of distant stars to search for planets outside our solar system. A drop in the light curve, as illustrated here, is a good indication that there's a planet orbiting the observed star. Credit: NASA Ames

The Kepler mission measures the brightness of stars. The data will look like an EKG showing the heart beat. Whenever a planet passes in front of its parent star as viewed from the spacecraft, a tiny pulse or beat is produced. From the repeated beats, we can detect and verify the existence of Earth-size planets and learn about the orbit and size of the planet. Video credit: NASA Ames and Dana Berry

Additionally, scientists have begun exploring the exospheres of exoplanets. By observing the spectra of the light that passes through an exosphere – similar to how we study Mercury’s exosphere – scientists are beginning to understand the evolution of exoplanet atmospheres as well as the influence of stellar wind and magnetic fields.

Watch It

Mercury will appear as a tiny dot on the sun’s surface and will require a telescope or binoculars with a special solar filter to see. Looking at the sun directly or through a telescope without proper protection can lead to serious and permanent vision damage. Do not look directly at the sun without a solar filter.

The transit of Mercury will begin at 4:12 a.m. PDT, meaning by the time the sun rises on the West Coast, Mercury will have been transiting the sun for nearly two hours. Fortunately, it will take seven and a half hours for Mercury to completely cross the sun’s face, so there will be plenty of time for West Coast viewers to witness this event. See the transit map to learn when and where the transit will be visible.

Don’t have access to a telescope, binoculars or a solar filter? Visit the Night Sky Network website for the location of events near you where amateur astronomers will have viewing opportunities available.

NASA also will stream a live program on NASA TV and the agency’s Facebook page from 7:30 to 8:30 a.m. PDT (10:30 to 11:30 a.m. EDT) -- an informal roundtable during which experts representing planetary, heliophysics and astrophysics will discuss the science behind the Mercury transit. Viewers can ask questions via Facebook and Twitter using #AskNASA.

Teach It

Here are two ways to turn the transit of Mercury into a lesson for students.

  • Exploring Exoplanets with Kepler - Students use math concepts related to transits to discover real-world data about Mercury, Venus and planets outside our solar system.
  • Pi in the Sky 3 - Try the "Sun Screen" problem on this illustrated math problem set that has students calculate the percentage drop in sunlight reaching Earth when Mercury transits.

Explore More

Transit Resources:

Exoplanet Resources:

TAGS: Transit, Transit of Mercury, Mercury, Venus, Sun, Exoplanets, Teach, Classroom Activities, Lessons,

  • Lyle Tavernier